Nitrogen Metabolism and Protein Digestion

Comprehensive vocabulary flashcards covering the biochemical principles of protein digestion, amino acid transport disorders, transamination, deamination, and hepatic ammonia detoxification.

Amino Acid Pool

The aggregate free amino acid reserve supplied by dietary intake, tissue protein degradation, and de novo synthesis of non-essential amino acids, utilized for protein synthesis, nitrogen-containing compounds, or catabolized into urea and carbon skeletons.

Positive Nitrogen Balance

A physiological state where dietary nitrogen ingestion exceeds nitrogen excretion ($\text{protein synthesis} > \text{protein degradation}$).

Negative Nitrogen Balance

A metabolic state where nitrogen excretion exceeds dietary nitrogen intake ($\text{protein synthesis} < \text{protein degradation}$).

Parietal Cells

Gastric epithelial cells that secrete hydrochloric acid ($\text{HCl}$) to denature proteins and activate zymogens, as well as intrinsic factor to facilitate vitamin $\text{B}_{12}$ absorption.

Chief Cells

Gastric gland cells responsible for secreting pepsinogen, the inactive precursor to the endopeptidase pepsin.

Pepsin

A gastric endopeptidase activated from pepsinogen by acidic $\text{pH}$ that initiates protein digestion by cleaving peptide bonds involving phenylalanine, tyrosine, and leucine.

Enteropeptidase (Enterokinase)

A duodenal brush border enzyme that initiates the pancreatic zymogen activation cascade by converting trypsinogen into active trypsin.

Trypsin

An active pancreatic protease that cleaves peptide bonds at the C-terminal of basic amino acids (lysine and arginine) and activates other inactive pancreatic zymogens.

Chymotrypsin

A pancreatic protease activated from chymotrypsinogen by trypsin that selectively cleaves peptide bonds containing aromatic and large hydrophobic amino acids.

Elastase

A pancreatic serine protease activated from its zymogen form by trypsin that selectively hydrolyzes peptide bonds involving serine.

Carboxypeptidase A

A pancreatic zinc exopeptidase activated from procarboxypeptidase A by trypsin that cleaves C-terminal amino acid residues involving aromatic amino acids.

Carboxypeptidase B

A pancreatic exopeptidase activated from procarboxypeptidase B by trypsin that hydrolyzes terminal peptide bonds of basic amino acids at the C-terminus.

Hartnup Disease

An autosomal recessive disorder caused by defective intestinal and renal neutral amino acid transporters, leading to neutral aminoaciduria and clinical features resembling pellagra due to tryptophan wasting.

Pellagra

A nutritional disorder resulting from niacin (vitamin $\text{B}_3$) or tryptophan deficiency, clinically characterized by the 4 Ds: Diarrhea, Dermatitis, Dementia, and Death.

COLA Transporter

A shared renal and intestinal dibasic carrier system responsible for the reabsorption of Cystine, Ornithine, Lysine, and Arginine.

Cystinuria

A hereditary autosomal recessive defect (prevalence $1:7000$) in the COLA transporter that impairs cystine reabsorption, leading to recurrent formation of hexagonal cystine calculi in urine.

Cystine

A dimeric amino acid molecule formed by two cysteine residues joined together by a covalent disulfide bond ($\text{S}-\text{S}$).

Cyanide-Nitroprusside Test

A qualitative diagnostic chemical urine test used to detect elevated levels of cystine in the urine.

Transamination

A reversible enzymatic reaction transferring an $\alpha$-amino group from an amino acid to an $\alpha$-ketoacid, enabling amino acid degradation or non-essential amino acid synthesis.

Pyridoxal Phosphate (PLP)

The active coenzyme form of vitamin $\text{B}_6$, essential for enzymatic transamination and decarboxylation reactions of amino acids.

Alanine Aminotransferase (ALT)

A transaminase that reversibly catalyzes the reaction: $\text{alanine} + \alpha\text{-ketoglutarate} \rightleftharpoons \text{pyruvate} + \text{glutamate}$.

Aspartate Aminotransferase (AST)

A transaminase that reversibly catalyzes the reaction: $\text{aspartate} + \alpha\text{-ketoglutarate} \rightleftharpoons \text{oxaloacetate} + \text{glutamate}$.

Glutamate Dehydrogenase

A mitochondrial enzyme that reversibly oxidatively deaminates glutamate to $\alpha$-ketoglutarate, releasing free ammonium ($\text{NH}_4^+$) using $\text{NAD}^+$ or $\text{NADP}^+$.

Glutaminase

An enzyme present in the kidneys and liver that catalyzes the irreversible deamination of glutamine to glutamate and free ammonium ($\text{NH}_4^+$).

Amino Acid Decarboxylation

The enzymatic removal of the $\alpha$-carboxyl group from an amino acid as $\text{CO}_2$, requiring PLP to generate physiologically active biogenic amines such as GABA, histamine, or dopamine.

Glucogenic Amino Acids

Amino acids whose carbon skeletons are catabolized to pyruvate or citric acid cycle intermediates that can enter gluconeogenesis.

Ketogenic Amino Acids

Amino acids whose catabolism yields ketone body precursors, specifically acetyl-CoA or acetoacetyl-CoA; leucine and lysine are purely ketogenic.

Glutamine Synthetase

A cytosolic enzyme that aminates glutamate using $\text{ATP}$ and $\text{NH}_4^+$ to synthesize glutamine, capturing excess nitrogen for nontoxic transport in blood.

Alanine-Glucose Cycle

A metabolic pathway where muscle tissue exports ammonia and pyruvate as alanine to the liver, where nitrogen is converted into urea and carbon is used for gluconeogenesis.

Periportal Hepatocytes

Liver parenchymal cells that handle high concentrations of portal ammonia primarily via the urea cycle, utilizing carbamoyl phosphate synthetase 1 ($\text{CPS-1}$).

Perivenous Hepatocytes

Downstream hepatocytes containing high-affinity glutamine synthetase with a very low $K_m$ for ammonia to scavenge any remaining ammonia before blood leaves the liver.